Fastest OM lens would have been the 55mm f1.2 (which was reputed not to be very good fully open, but excellent stopped down to f2). The 50mm f1.4 was the fastest standard, and could have been used open.
OM-1 has shutter settings down to 1s (Plus B, of course); Later OMs had up to 4min auto exposure, but that is way beyond hand held. With a standard lens, it would be reasonable to have 1/30s as the longest exposure that is practicable for any sensible results.
Coming back to film, Ilford XP1 was launched in 1980, so was a plausible medium. It used a different technology to other B/W films, and had the latitude of colour print film (an advantage and a disadvantage in use). I have pushed it to 3200 ASA successfully for interiors of theatres. But, while not grainy, it was a bit of a strain for detail. If they showed the negatives in that classic darkroom shot after dev, the negs look distinctly sepia or purplish, depending on the process.
So, 1/30s @ f1.4 ISO3200 I think are the practical limits.
I'll let someone else do the maths on how dark that handles. Ftr, I used my 28-70mm f3.5-4.5 zoom for the theatres. Some results were better than others :/